Wireless local area network (WLAN) technology has become very popular in recent years, because of its advantage in price and bandwidth. Nowadays, wireless LAN is mainly used for Internet access, but real-time applications like Voice-over-IP (VoIP) and video-on-demand (Vod) are identified as likely future applications for wireless LAN. To support such new applications, the IEEE 802.11e working group has been discussing a new 802.11 medium access control (MAC) layer protocol. Standard 802.11e is described in “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Medium Access Control (MAC) Quality of Service (QoS) Enhancements,” IEEE Standard 802.11e/D12.0, November 2004. IEEE Standard 802.11e is also referred to in this specification simply as “802.11e.”
One of the most important keys to efficiently use the WM (wireless medium) is selection of access points (APs) with which stations (STAs) associate. In the case of the environment of multiple APs, if STAs have no strategy for AP selection, then the wireless bandwidth will be inefficiently used. Strategy for AP selection is often dependent upon a vendor's implementation. Furthermore, if the WLAN networks support 802.11e, it is necessary to consider how to guarantee quality of service (QoS), and wireless bandwidth must be appropriately allocated to multimedia traffic.
The IEEE 802.11e MAC is a standard to support Quality of Service (QoS). 802.11e Hybrid Coordination Function (HCF) can support QoS in 802.11 networks. The HCF provides both a content-based channel access, called enhanced distributed channel access (EDCA), and a controlled channel access, referred to as HCF controlled channel access (HCCA). A QoS-enhanced station (QSTA), which requires a strict QoS support, is allowed to transmit QoS requirement frames (ADDTS request frames) to the QoS-AP (QAP) in order to construct a Traffic Stream (TS). The QAP takes an admission control to accept or deny such a request from QSTAs.
If a request of TS is accepted, the QAP schedules how long QSTAs occupy the wireless medium or when QSTAs should transmit or receive frames. Moreover, in the field of WLAN, radio resource management (RRM) technology has many attentions due to the requirement of efficient use of radio resource. In the environment of multiple APs, RRM becomes very important. If each AP is deployed without any strategies, radio resource is inefficiently utilized. Besides AP deployment, when there are multiple APs (for example, in enterprise environment), STAs has to select one of them to associate with. If those STAs have no strategy with regard to radio resource, the wireless bandwidth will not be efficiently used. Furthermore, in order to support QoS in an environment where multiple APs exist, how to allocate radio resource to real-time traffic must be considered.
Currently, there are already some prior art documents that introduce algorithms for judging access points. Some of those prior art patents utilize algorithms that are based upon current loads and signal characteristics.
For example, consider Engwer et al. (U.S. Pat. No. 5,987,062) titled “Seamless roaming for wireless local area networks.” According to the Engwer patent, a wireless local area network allows roaming of a mobile unit to allow it to serially associate with a number of access points of the network fixed backbone. This roaming is supported by an improved measurement of communications link quality, which includes calculating a mean error free length of a test pattern broadcast by each access point and received by a mobile unit. The test pattern is a digital data message. Thus an accurate measurement of link quality is provided which allows a mobile unit to determine whether it should change its association to another access point having improved communications link quality. Further, a load balancing process is provided to balance the communications load amongst a variety of access points, by allowing mobile units also to switch their association with access points in accordance with a current total data rate at any given access point and also considering the number of currently high data rate mobile units associated with a particular access point at any one time.
Also, consider Kostic et al. (U.S. patent application Ser. No. 20030139197) titled “WLAN having load balancing based on access point loading.” According to the Kostic application, a network having distribution of access point loading includes access points to which mobile stations can associate themselves based upon access point beacon signal levels and loading levels for the various access points. A mobile station receives beacon signals from various access points and determines a signal strength for the received beacon signals. The mobile station also receives access point loading information from the access points. The mobile station associates with an access point based upon the access point beacon signal strengths and the access point loading information.
Furthermore, consider Wang (U.S. application Ser. No. 20050053046) titled “QoS based load-balance policy for WLAN.” According to the Wang application, a load balancing method for a wireless local area network (LAN) which has a plurality of access points is provided in the present invention. The load balancing decision is made by a load balancing module according to traffic conditions and bandwidth availability of each traffic priority class based on a corresponding class of service. The load balancing module is a centralized module which the load balancing decision is determined by the centralized module, or is determined through information exchanges among distributed load balancing modules of the access points. The load balancing decision also takes into consideration of VLANs per VLAN tag basis. Each of the VLANs is defined with the corresponding traffic priority class.
As mentioned, IEEE standard 802.11e defines two access mechanisms for supporting QoS: enhanced distributed channel access (EDCA) and hybrid coordination function controlled channel access (HCCA). 802.11e supports admission control under both EDCA and HCCA. According to the 802.11 standard, STAs are responsible for deciding an AP with which they associate, and a strategy of AP selection is dependent on vendors' implementation. The most familiar way to select AP is to use a RSSI (Received Signal Strength Indicator). However, if STAs associate with an AP whose RSSI value is the highest, then the AP's network may suffer from a heavy load. Thus, some research has taken into account RSSI and also the number of associated STAs for the AP selection decision. This is described in an article by Papanikos et al., “A study on dynamic load balance for IEEE 802.11b wireless LAN,” in Proc. of COMCON 2001, 2001. If only best effort traffic is considered, those prior art methods might be acceptable. But, if the AP supports 802.11e, those prior art methods do not maintain QoS, because they do not distinguish traffic types in a WLAN network. Thus, QoS must be carefully treated in the QoS-supported WLAN. Thus, there is a need for this kind AP selection with QoS in mind.
Physical transmission rate is important for efficiently utilizing the radio resource. Taking into account RSSI, a link adaptation method to decide physical transmission rate was proposed by Pavon et al., “Link Adaptation Strategy for IEEE802.11 WLAN via Received Signal Strength Measurement,” in Proc. of IEEE ICC2003, June 2003.
The aforementioned prior art, including Engwer, Kostic, Wang, Papanikos, and Pavon, is entirely incorporated herein by reference.